1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing device.
2. Description of the Related Art
As a fixing device used in an image forming apparatus such as a copier, a printer, a facsimile, or an MFP including any of these, a device including a belt-shaped or film-shaped fixing member having an endlessly moving surface and having lower heat capacity, and a heat source for heating the fixing member has been known (for example, see Japanese Patent Application Laid-open No. 2007-334205, Japanese Patent Application Laid-open No. 2007-233011, Japanese Patent Application Laid-open No. 2010-032625, and Japanese Patent Application Laid-open No. 2010-217257). The provision of the fixing member with lower heat capacity can drastically reduce the energy required for heating the fixing member and shorten the warm-up time or first printing time. Here, the warm-up time refers to the time required for increasing the temperature of the fixing member when powered on, for example, from normal temperature to a predetermined printable temperature (reload temperature). The first printing time refers to the time after the reception of a printing request through printing preparation and printing operation until the completion of discharge of paper.
In the conventional fixing device described above, for surely heating a recording medium such as paper to which an image is transferred, a wider range of the fixing member is heated than a range of the fixing member where the printing medium passing a nip portion is in contact in a width direction orthogonal to the surface moving direction. Then, the temperature rise in a part of the range where the surface of the fixing member is heated, that is not in contact with the recording medium, i.e., a portion that corresponds to the outside of both ends of the recording medium in a width direction thereof becomes larger than that in a portion that is in contact with the recording medium; thus, a high-temperature portion having high temperature is formed locally on the surface of the fixing member. The high-temperature portion on the surface of the fixing member is easily formed particularly in the fixing device in which the fixing member with an endless belt form is directly heated as described in Japanese Patent Application Laid-open No. 2007-233011. The formation of the high-temperature portion on the surface of the fixing member might causes the fixing failure such as a hot-offset image or gloss unevenness when the passage of a recording medium with small width is followed by the passage of a recording medium with large width through a nip portion.
Meanwhile, in the image forming apparatus including the fixing device, there has been a request for shortening the image forming time required for forming an image on the recording medium.
Therefore, there is a need for an image forming apparatus that is capable of preventing the occurrence of fixing failure due to the inhomogeneous distribution of temperature of the fixing member in a width direction of the recording medium, which is orthogonal to a recording medium conveying direction, and is capable of shortening the image forming time required for forming the images on these recording media in the case where an image is formed on a recording medium with a size in the width direction, at a nip portion of the fixing device, less than or equal to a predetermined size, and then, an image is formed on a recording medium with the size in the width direction larger than the predetermined size.